Hydraulic pumps of the free-type are circulated downhole through a power oil string where the pump intake is seated on a shoe, so that formation fluid can flow through the seat and into the pump intake. At the same time, provision must be made so that power fluid does not flow between the interior of the power fluid string and the exterior of the pump housing. This is achieved by a pump cavity having a collar located therein, with there being seal means positioned between the pump housing and the collar, as seen for example in Roeder U.S. Pat. No. 4,088,328, to which reference is made, including the prior art cited therein.
Often a borehole extends vertically downhole and then is slanted back towards the horizontal. For example, directional drilling is usually employed on offshore platforms, and there may be more than 15 or 20 slanted boreholes associated with a single platform. When a free-type pump is circulated downhole in a slanted borehole, the downward travel causes excessive wear on the bottom side of the o-ring seals as the traveling pump rubs against the tubing interior, for the reason that the pump gravitates downwardly against the lower inside surface of the tubing string. Moreover, after the pump is seated into operative position, excessive wear continues on the lower side of the seal assembly, due to induced movement of the operating pump and gravitational force. Leakage of power fluid across a worn seal assembly progressively reduces the pump efficiency.
It would therefore be desirable to provide a free-type, downhole, hydraulically actuated pump with a seal assembly which is not subjected to wear during its travel into and out of the borehole, and which furthermore centralizes the pump within the pump cavity in a manner which reduces wear on the seals, as well as stabilizing the pump while the pump is in the seated position. A seal gland assembly which achieves this desirable goal is the subject of the present invention.